The disclosure relates to a brake disk, the friction surface of which has a surface coating, having the features described below.
Brake disks for disk brakes of motor vehicles are known. Typically, they are composed of metal, usually of gray cast iron, i.e. of cast iron containing graphite flakes, and typically also of steel. Also known are brake disks made of carbon fiber-reinforced plastic, these being used in motorsport because of their low weight and being restricted essentially to motorsport because of their high wear.
To reduce wear, a wear-reducing surface coating on friction surfaces of the brake disk is known, especially in the case of brake disks made of gray cast iron. Friction surfaces are the brake disk surfaces in the form of perforated circular disks against which the friction brake linings are pressed during braking.
Known wear-reducing surface coatings contain particles of chromium carbide and/or tungsten carbide, i.e. metal-like carbides. The carbide particles are embedded in a metallic matrix composed of nickel or cobalt, for example. The surface coating is applied by thermal spraying, e.g. flame spraying or arc spraying.
As already mentioned, the surface coating increases the wear resistance of the brake disk, which is a desirable effect. At the same time, the surface coating prolongs the wearing-in period of the brake disk owing to its relatively high wear resistance in comparison with the uncoated brake disk. In the case of a new brake disk, wear during braking alters the braking properties, in particular a friction coefficient of the brake disk. After a certain period, the wearing-in period, or after a certain number of braking operations or a certain initial wear, the brake disk reaches a stable state, and its braking properties, in particular its friction coefficient, no longer change or at least change only insignificantly or not noticeably. In order to give a driver a feeling of safety when braking, the wearing-in period should be as short as possible, i.e. a brake disk should exhibit its stable braking state after as few braking operations as possible. Uncoated gray cast iron brake disks reach the stable state after about 30 normal braking operations in road traffic, for example, that is to say not very gentle and also not excessively strong braking operations. Tests have shown that brake disks with a wear-reducing surface coating on the friction surfaces thereof have still not reached the stable state after 120 braking operations, i.e. four times the number of braking operations.